The tyrosine kinase inhibitor dasatinib dysregulates bone remodeling through inhibition of osteoclasts in vivo.

Dasatinib is a potent tyrosine kinase inhibitor that is used to treat chronic myeloid leukemia in patients resistant or intolerant to imatinib mesylate. While designed to inhibit Abl and Src kinases, dasatinib shows multitarget effects, including inhibition of the macrophage colony-stimulating factor (M-CSF) receptor c-fms. We have shown previously that dasatinib abrogates osteoclast formation and activity in vitro owing, in part, to its specificity for c-fms. In this study we examined whether dasatinib could significantly alter bone volume in a model of physiologic bone turnover. Sprague-Dawley rats were administered dasatinib (5 mg/kg/day) or vehicle by gavage or zoledronic acid (ZOL; 100 microg/kg/6 weeks) subcutaneously. Following 4, 8, and 12 weeks of treatment, serum biochemical, bone morphometric, and histologic analyses were performed. Whole-body bone mineral density and tibial cortical thickness where unchanged in the dasatinib- or ZOL-treated animals relative to controls. However, micro-computed tomographic (microCT) analysis of cancellous bone at the proximal tibias showed that trabecular volume (BV/TV) and thickness (Tb.Th) were increased in dasatinib-treated animals at levels comparable with those of the ZOL-treated group. These changes were associated with a decrease in osteoclast numbers (N.Oc/B.Pm) and surface (Oc.S/BS) and decreased serum levels of the osteoclast marker c-terminal collagen crosslinks (CTX-1). Mineral apposition rate (MAR), bone-formation rate (BFR), and levels of the serum osteoblast markers osteocalcin and N-terminal propeptide of type I procollagen (P1NP) were not altered significantly in the dasatinib-treated animals relative to controls. These studies show that dasatinib increases trabecular bone volume at least in part by inhibiting osteoclast activity, suggesting that dasatinib therapy may result in dysregulated bone remodeling.

Dysregulation of bone remodeling by imatinib mesylate.

Imatinib mesylate is a rationally designed tyrosine kinase inhibitor that has revolutionized the treatment of chronic myeloid leukemia and gastrointestinal stromal tumors. Although the efficacy and tolerability of imatinib are a vast improvement over conventional chemotherapies, the drug exhibits off-target effects. An unanticipated side effect of imatinib therapy is hypophosphatemia and hypocalcemia, which in part has been attributed to drug-mediated changes to renal and gastrointestinal handling of phosphate and calcium. However, emerging data suggest that imatinib also targets cells of the skeleton, stimulating the retention and sequestration of calcium and phosphate to bone, leading to decreased circulating levels of these minerals. The aim of this review is to highlight our current understanding of the mechanisms surrounding the effects of imatinib on the skeleton. In particular, it examines recent studies suggesting that imatinib has direct effects on bone-resorbing osteoclasts and bone-forming osteoblasts through inhibition of c-fms, c-kit, carbonic anhydrase II, and the platelet-derived growth factor receptor. The potential application of imatinib in the treatment of cancer-induced osteolysis will also be discussed.

Long-term imatinib therapy promotes bone formation in CML patients.

Imatinib inhibits tyrosine kinases important in osteoclast (c-Fms) and osteoblast (platelet-derived growth factor receptor [PDGF-R], c-Abl) function, suggesting that long-term therapy may alter bone homeostasis. To investigate this question, we measured the trabecular bone volume (TBV) in iliac crest bone biopsies taken from chronic myeloid leukemia (CML) patients at diagnosis and again after 2 to 4 years of imatinib therapy. Half the patients (8 of 17) showed a substantive increase in TBV (> 2-fold), after imatinib therapy, with the TBV in the posttreatment biopsy typically surpassing the normal upper limit for the patient's age group. Imatinib-treated patients exhibited reduced serum calcium and phosphate levels with hypophosphatemia evident in 53% (9 of 17) of patients. In vitro, imatinib suppressed osteoblast proliferation and stimulated osteogenic gene expression and mineralized-matrix production by inhibiting PDGF receptor function. In PDGF-stimulated cultures, imatinib dose-dependently inhibited activation of Akt and Crk-L. Using pharmacologic inhibitors, inhibition of PI3-kinase/Akt activation promoted mineral formation, suggesting a possible molecular mechanism for the imatinib-mediated increase in TBV in vivo. Further investigation is required to determine whether the increase in TBV associated with imatinib therapy may represent a novel therapeutic avenue for the treatment of diseases that are characterized by generalized bone loss.

Tumor angiogenesis is associated with plasma levels of stromal-derived factor-1alpha in patients with multiple myeloma.

PURPOSE: Multiple myeloma is an incurable hematologic malignancy characterized by increased bone marrow angiogenesis and extensive lytic bone disease. We have previously shown that elevated levels of stromal-derived factor-1alpha (SDF-1alpha) in peripheral blood plasma are associated with osteolysis in multiple myeloma patients. We have now examined whether SDF-1alpha levels also correlate with angiogenesis. EXPERIMENTAL DESIGN: We examined the contribution of multiple myeloma plasma cell-derived SDF-1alpha in the stimulation of in vitro angiogenesis using a tube formation assay. We also collected trephine and peripheral blood plasma samples from patients with multiple myeloma to analyze microvessel density and SDF-1alpha levels, respectively. RESULTS: We show that multiple myeloma plasma cell line-derived conditioned medium containing SDF-1alpha stimulates in vitro angiogenesis. In addition, in a large cohort of patients with multiple myeloma and its precursor condition monoclonal gammopathy of undetermined significance, we confirm previous findings that plasma cell burden correlates with both angiogenesis and plasma levels of SDF-1alpha. We now extend these observations and show the novel finding that peripheral blood plasma levels of SDF-1alpha positively correlate with the degree of bone marrow angiogenesis in multiple myeloma and monoclonal gammopathy of undetermined significance patients. CONCLUSIONS: High levels of SDF-1alpha produced by multiple myeloma plasma cells promote osteolysis and bone marrow angiogenesis. Therefore, we propose that inhibition of SDF-1alpha may be an effective mechanism by which angiogenesis and osteolysis can be reduced in multiple myeloma patients.

Imatinib as a potential antiresorptive therapy for bone disease.

Osteoclasts (OCs) are large multinucleated cells derived from progenitor cells of the monocyte-macrophage lineage. Signal transduction via the macrophage-colony-stimulating factor (M-CSF) receptor, c-fms, is essential for OC formation. Since we have previously demonstrated inhibition of c-fms by imatinib, we examined the effect of imatinib on OC formation and activity. OC formation was not affected by concentrations of 1.0 microM imatinib and lower, but was reduced by 75% at 3.0 microM imatinib. In contrast, both the area of resorption and the number of resorption lacunae were reduced by 80% at 0.3 microM imatinib, and no resorption was observed at concentrations above 3.0 microM. A dose-dependent decrease in receptor activator of nuclear factor kappaB (RANK) expression was observed in OCs when cultured in the presence of imatinib, providing a mechanism for the decrease in OC function. In vivo analysis of the effect of imatinib on OC activity in adult mice following 8 weeks of imatinib treatment also demonstrated a decrease in OC activity. These results suggest that imatinib may have therapeutic value as an antiosteolytic agent in diseases such as osteoporosis, metastatic bone disease, and multiple myeloma.

Inhibition of c-fms by imatinib: expanding the spectrum of treatment.

Imatinib is a selective protein tyrosine kinase inhibitor currently used in the treatment of chronic myeloid leukaemia (CML). It specifically suppresses the growth of bcr-abl expressing CML progenitor cells by blocking the ATP-binding site of the kinase domain of bcr-abl. Imatinib also inhibits the c-abl, platelet derived growth factor receptor (PDGFR), abl-related gene and stem cell factor receptor, c-kit, protein tyrosine kinases. It is through inhibition of c-kit that imatinib is also used clinically in the treatment of gastrointestinal stromal tumours. We have recently demonstrated that imatinib also specifically targets the macrophage colony stimulating factor receptor, c-fms, at therapeutic concentrations. Although this finding has important implications with regard to potential side effects in patients currently receiving imatinib therapy, these results suggest that imatinib may also be useful in the treatment of diseases where c-fms is implicated. This includes breast and ovarian cancer and inflammatory conditions such as rheumatoid arthritis. We also speculate that imatinib may be used in diseases where bone destruction occurs due to excessive osteoclast activity, such as in the haematologic malignancy, multiple myeloma.

Macrophage colony-stimulating factor receptor c-fms is a novel target of imatinib.

Imatinib is a tyrosine kinase inhibitor that suppresses the growth of bcr-abl-expressing chronic myeloid leukemia (CML) progenitor cells by blockade of the adenosine triphosphate (ATP)-binding site of the kinase domain of bcr-abl. Imatinib also inhibits the c-abl, platelet-derived growth factor (PDGF) receptor, abl-related gene (ARG) and stem-cell factor (SCF) receptor tyrosine kinases, and has been used clinically to inhibit the growth of malignant cells in patients with CML and gastrointestinal stromal tumors (GISTs). Although initially considered to have minimal effects of normal hematopoiesis, recent studies show that imatinib also inhibits the growth of some nonmalignant hematopoietic cells, including monocyte/macrophages. This inhibition could not be attributed to the known activity profile of imatinib. Here, we demonstrate for the first time that imatinib targets the macrophage colony-stimulating factor (M-CSF) receptor c-fms. Phosphorylation of c-fms was inhibited by therapeutic concentrations of imatinib, and this was not due to down-regulation in c-fms expression. Imatinib was also found to inhibit M-CSF-induced proliferation of a cytokine-dependent cell line, further supporting the hypothesis that imatinib affects the growth and development of monocyte and/or macrophages through inhibition of c-fms signaling. Importantly, these results identify an additional biologic target to those already defined for imatinib. Imatinib should now be assessed for activity in diseases where c-fms activation is implicated, including breast and ovarian cancer and inflammatory conditions.

Imatinib inhibits the functional capacity of cultured human monocytes.

Imatinib is a tyrosine kinase inhibitor that has been reported to specifically inhibit the growth of bcr-abl expressing chronic myeloid leukaemia progenitors. This drug functions by blocking the ATP-binding site of the kinase domain of bcr-abl, and has also been found to inhibit the c-abl, platelet-derived growth factor receptor, ARG and stem cell factor receptor tyrosine kinases. Reports have recently emerged demonstrating that imatinib also inhibits the growth of non-malignant haemopoietic cells. Here, we demonstrate that concentrations of imatinib within the therapeutic dose range inhibit the function of cultured monocytes (CM) from normal donors. A decrease in the response of CM to LPS was observed morphologically and functionally, with CM grown in the presence of imatinib showing decreased pseudopodia formation and inhibition of IL-6 and TNF-alpha production following LPS stimulation. Imatinib also reduced the ability of M-CSF and GM-CSF stimulated CM to phagocytose zymosan particles, with uptake of non-opsonized zymosan by M-CSF stimulated CM (M-CM) being most affected. M-CM that had been cultured in the presence of imatinib were also impaired in their ability to stimulate responder cells in a mixed lymphocyte reaction. These results demonstrate that human monocytes cultured in the presence of imatinib are functionally impaired, and suggest that imatinib displays inhibitory activity against other kinase(s) that play a role in monocyte/macrophage development.